The overall objective of the proposed research is to elucidate in molecular terms the mechanisms by which interferon acts to affect a wide variety of fundamental viral and cellular processes. The specific aims are: (1) To further purify by chromatographic and electrophoretic techniques the interferon-mediated ribosome-associated polypeptide(s) present in interferon-treated mouse ascites and L cells and normally absent in untreated murine cells, and then to determine the biochemical and biophysical properties of the purified polypeptide(s), (2) To elucidate the precise biochemical mechanism by which the translation of viral messenger RNA catalyzed by cell-free protein synthesizing systems prepared from untreated ascites cells is inhibited by the purified interferon-mediated factors. This will be performed by kinetic analysis of reovirus mRNA translation in the presence and absence of the purified factor together with added reovirus ds-RNA, ascites tRNA, specific antibiotic inhibitors, and/or various nucleotides, as well as by binding studies utilizing the purified, radioactively-labeled factor and various components of the protein synthesis machinery. (3) To resolve whether the inhibitory activity of the host coded interferom-mediated factor is directed specifically toward viral messenger RNAs, or whether the translation of certain classes of cellular messenger RNAs are also selectively affected; and, whether deletion of regions of the 5'-terminal untranslated region of vifal mRNA alters the inhibitory activity of the factor. This will be performed by studying the kinetics of translation in vitro of several different purified viral and cellular messenger RNAs both individually and in competition with each other, as well as the translation in a coupled system of viral mRNA derived from wild type and viable deletion mutants of SV40, (4) To determine whether the interferon-mediated inhibition of SV40 multiplication and of reovirus multiplication is exerted at a common or different level(s) of macromolecular synthesis. This will be determined in a common host cell by examining the effect of interferon on early viral RNA and viral protein synthesis with wild type and mutant virus strains.